The Internal Anatomy of Spiders 



completely surrounding it, is a thin-walled sac, the pericardium 

 (p). Between the pericardium and the heart is the pericardial 

 cavity (pc). The first one or two pairs of cardiac ligaments are 

 larger than the others and function as pulmonary veins (pv). 

 Each of the above-mentioned parts is described more in detail 

 below. 



The ostia of the heart. — The wall of the heart is pierced by 

 several pairs of openings by which the blood is received into 

 the heart; these are the ostia (Fig. 158, o). In the four-lunged 

 spiders there are four pairs of ostia (Fig. 158, 1); in most of the 

 two-lunged spiders there are three pairs (Fig. 158, 2); but in some 

 of the two-lunged spiders, as in Dysdera (Fig. 158, 3), the number 

 is reduced to two. 



The diverticula of the heart. — As indicated above, the cavity 

 of the heart is more or less expanded at regular intervals; and 

 these expansions form paired diverticula. In the more generalized 

 spiders, six pairs of these diverticula can be recognized. The 

 four pairs of ostia of the heart in these spiders are in the walls 

 of the first, second, third, and fifth pairs of diverticula respectively 

 (Fig. 158, 1); the fourth pair of diverticula are midway between 

 the third and fourth pairs of ostia, and bear only vestigial liga- 

 ments; the sixth pair is near the caudal end of the heart. In 

 more specialized spiders the number of diverticula is more or 

 less reduced; those which are most constantly prominent are the 

 first, third, and fifth pairs, in the walls of which are the three 

 pairs of ostia characteristic of most two-lunged spiders; the sixth 

 pair are also usually well-marked, as from them arise a pair of 

 arteries. 



The aorta. — The aorta (Fig. 1 58, a) is the forward prolon- 

 gation of the heart. It extends through the pedicel of the abdo- 

 men into the cephalothorax (Fig. 143). As it enters the 

 cephalothorax, it gives off some small branches which ramify 

 among the muscles of the posterior part of the cephalothorax. 

 It then extends forward to a point near the sucking stomach, 

 where it divides into two trunks, one passing forward on each 

 side between the sucking stomach and the caecal ring. Each of 

 these trunks sends branches to the appendages, to the eyes, 

 to the muscles, and to other organs of its side. This account of 

 the course of the branches of the aorta is based on the figures and 

 descriptions given by Causard ('96) and those by Petrunkevitch 



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